Go on to your account and start the Cancellation process for Cinema. You will immediately be offered a discount. Accept it and exit the cancellation.
It’s all automated - you don’t have to haggle - and it would have worked from month 1.
Though the algorithm by which the offers are made, and the best time to cancel to get the optimum discount, are closely guarded secrets.
Except I think that now is always the best time….
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.
If you take an offer that comes with a ‘free trial’ of Boost or another Membership, Cancel them straight away. You will still get the full duration of the trial, and if you decide you do want to continue with one or another of the trial products, just revisit the Cancel, and UnCancel it.